94 THE MIGRATION OF BIRDS 



merely to show that apparently unfavourable 

 winds may be really favourable. 



Under ordinary circumstances are we justified 

 in saying that birds make use of the winds blowing 

 with a certain force at the point of departure, or 

 that they ignore them ? Certainly we cannot judge 

 by either the force or direction of the wind at the 

 point of arrival, as Mr Clarke points out. The bird 

 may have dropped into most adverse currents. 



In Hungary, where migration has been very 

 carefully studied, we find evidence supporting 

 Mr Clarke's theory, and yet giving it a slightly 

 different complexion. Low atmospheric pressure, 

 depression (the warm cyclonic conditions of spring) 

 very often shows the greatest rate in the arrival of 

 the swallow. If there is a centre of depression 

 west of Hungary, and its path is directed north or 

 north-east, swallows appear in crowds. The fair 

 side of the depression, with its warm southerly 

 winds, is therefore favourable. A list of twelve 

 other birds, which also appear in spring under these 

 conditions in greatest numbers, is added. The 

 ' bad ' side, with cool northerly winds causes 

 delays in the arrival of these thirteen species. 

 The depressions often have a sphere of influence 

 extending so far as North Africa, so that birds, on 

 the fair side, can cross the Mediterranean with 

 southerly winds all the way (31). 



